Principle of lens coating 

Why do lenses and lenses need coating？This is because any object has a reflection effect on light，Even colorless and transparent glass is no exception，The difference lies in whether the angle of the light will create a reflection effect。For ideal lenses，Light can completely pass through the lens，And correctly on the film or CCD Fully focused on。however，In fact, it is，Each type of lens is limited by its own physical factors，Causing the generation of aberrations（See previous lecture for details），therefore，There are many『problem』The lens formed by the combination of lenses cannot theoretically allow all kinds of angles of light to pass through completely。Taking lanthanum oxide optical glass as an example，Its transmittance can reach 90%the above，remaining 10% It will reflect off，Create dazzling light。To make up for this deficiency，Later lens researchers developed a coating on the surface of the lens to increase its transparency。 

Uncoated lens，The transmittance of its lenses is relatively low，The reflection on the surface of the lens is quite severe，be called“Whitehead”，Strong reflection of various light rays in the spectrum，Therefore, the overall color of the reflection turns white。   

Single layer coated lens，The reflection on the surface of its lens is weak，It can greatly increase the transmittance of yellow green light in the middle of the spectrum，Only the red and blue light at both ends of the spectrum are reflected，Therefore, the reflection is generally light blue purple。 

Multi layer coated lens，The transparency of its lens is extremely high，The direct reflection on the surface of the lens is very weak，Cover the rear cover of the lens，Looking directly at the mirror glass against the light，I only saw inside the camera“Very dark”，Only by observing from the side of the lens can colored reflections be seen，This kind of reflection is mostly deep red（Significantly increase blue light transmittance）、Dark Blue（Significantly increase red light transmittance）、dark yellow（Significantly increase the transparency of blue-green light）And dark green，The deep green coating can simultaneously increase the transmittance of blue and red light at both ends of the spectrum，Only the yellow green light in the middle of the spectrum is reflected back，Because the transmittance curve of this anti reflective membrane has a red color、Two blue anti reflection peaks，Like two humps，Therefore, it is also known as“Bimodal membrane”。Multi layer coated lens，It is impossible for all lenses to be coated with the same type of anti reflective film，Otherwise, this lens will experience color cast。 

therefore，Different lenses for each lens，It depends on the material used for the lens and its absorption of different colors of light，Coating anti reflective films with different characteristics separately，Match with each other，Can increase the overall transmittance of the lens，It can also achieve a balance in the transmittance of colored light through the lens，Achieve neither blue nor red bias。therefore，Different materials of lenses need to be coated with anti reflective films with different characteristics，So the color of the reflective lens cannot be the same。  

    The function of coating is to-Can reduce the reflection of light，Improve the brightness of the image，And it can eliminate the halo phenomenon caused by light reflection，Get a good contrast。UVThe mirror will not affect the use of the lens，Installed itUVMirrors can better protect the lens。  

    At every interface between the lens and the air,Light will reflect4%-7%.like this,It reduces the amount of light passing through the lens.The reflected light will undergo multiple reflections and refractions,form"ghost image",Simultaneously generate halo,Reduce the contrast of the image.In a single lens,Reflection loss of light in8%-14%between.Light can be lost in a lens with a three piece structure25%the above.This reduces the actual amount of light passing through,Reduced effective aperture.In modern lenses,More than ten lenses are common.In such a shot,If there is no film added,Not only does the amount of light flow decrease to the point where it cannot be used,Internal reflection halo also makes imaging unacceptable.After adding film,Can reduce reflection of light within a certain wavelength range,Increase the amount of light passing through,Thus, the lens with multiple lenses has reached a practical level.